Well, a cool plot for a Witcher RWBY crossover fanfiction might involve the Witcher's magic and potions being introduced to the RWBY universe. For example, a potion that gives the RWBY characters temporary enhanced senses like a Witcher. Or perhaps a situation where a powerful Witcher villain comes to the RWBY world and the teams have to combine their skills to defeat him. The different fighting styles of the Witcher and RWBY characters would make for some really exciting battle scenes.
In a Witcher RWBY crossover fanfiction, a great plot could be centered around a shared threat that exists in both worlds. Say, a group of mages from the Witcher world who want to use the power of the Grimm in RWBY for their own evil purposes. The RWBY characters could use their teamwork and unique abilities, while Geralt and other Witchers would bring their combat experience and knowledge of magic. Weiss could learn from the Witcher mages, and in return, Geralt could learn about the Dust in the RWBY world.
One idea could be a crossover with the Lord of the Rings. Geralt could find himself in Middle - earth, facing off against orcs and perhaps teaming up with Aragorn. Another is a crossover with Harry Potter. Maybe Geralt is called to Hogwarts to deal with a magical beast that the wizards can't handle. A third idea is a crossover with Game of Thrones, where Geralt has to navigate the complex politics and battles of Westeros.
One idea could be that Geralt gets transported to the wizarding world. He could be mistaken for a very strange kind of wizard at Hogwarts. Maybe he's enlisted to help fight some dark magic that the regular wizards can't handle because of his unique skills in dealing with monsters. Another plot could involve Harry Potter being sent to the Witcher's world. He'd have to learn how to use a sword and deal with the more brutal and less rule - bound magic there. And a third idea is that a powerful dark force is threatening both worlds, so the Witchers and the wizards from Harry Potter's world have to team up. The Witchers could teach the wizards about physical combat and the wizards could share their more sophisticated spells with the Witchers.
A possible plot is that a portal opens between the Doom universe and RWBY's. Some of the Doom demons start spilling into Remnant, the world of RWBY. Ruby and her team initially mistake them for a new, extremely powerful type of Grimm. As they try to fight them, they realize they need different strategies. Meanwhile, the Doom Slayer follows the demons through the portal to stop them. Eventually, the two groups meet and learn from each other's combat styles to defeat the invading demons.
Well, another plot could involve Sans and Papyrus coming to the RWBY world. Papyrus might be fascinated by the Grimm and try to befriend them, while Sans could use his powers to help the RWBY team in their battles against Salem. He could also share his knowledge of timelines with Ozpin.
One idea could be the Overwatch heroes being transported to the RWBY world. They could team up with Team RWBY to fight a new common enemy, like a powerful Grimm that is immune to normal attacks but vulnerable to Overwatch tech. Another plot could involve a character exchange, where a RWBY character like Ruby goes to the Overwatch world and learns about their technology and fighting styles, and then brings back new knowledge to her own world.
One idea could be Cinder being transported to an alternate universe within the RWBY world where the roles are reversed. For example, she could find herself in a world where she is a huntress in training, facing the same challenges as the main characters but from a different perspective. Another plot could involve a team - up between Cinder and a group of RWBY characters against a common, greater enemy that threatens both their goals and the world. Maybe a new, powerful Grimm that has abilities they've never seen before.
One idea could be that the Dragonborn is somehow transported to the RWBY world. They could use their shouts to combat the Grimm. For example, 'Fus Ro Dah' could send Grimm flying back, and the local Huntsmen and Huntresses are both amazed and a bit wary of this new power.
One interesting plot idea could be that the Joker somehow gets transported to the RWBY universe. He might start causing chaos in Vale, tricking the villains and heroes alike. For example, he could convince Roman Torchwick to do his bidding in a more extreme way, which would then draw the attention of Team RWBY. Another idea is that he teams up with Cinder Fall, but his chaotic nature eventually causes their alliance to crumble as he doesn't follow her grand plans in the way she expects.
A more complex plot could be centered around the concept of the Soul Society in the RWBY world. What if the RWBY characters find out about the afterlife as it exists in Bleach? They might try to find a way to communicate with their fallen comrades who are in the Soul Society. This could open up a whole new dimension of character development and exploration of themes like life, death, and the afterlife.
One idea could be that the Titans from Titanfall end up in the world of RWBY. They could team up with the Huntsmen and Huntresses to fight a new, powerful Grimm threat that is too much for either side to handle alone. The Pilots could share their advanced combat skills with the RWBY characters, and in return, the RWBY characters could teach the Pilots about the use of Aura and Semblances.